This invention relates generally to electrostatically operated microelectromechanical systems (MEMS) devices and, more particularly, to radio frequency (RF) capacitive MEMS switches, and other devices utilizing dielectric materials.
RF MEMS capacitive switches have many useful applications for military and commercial RF and microwave applications. An RF MEMS capacitive switch comprises a movable metal membrane (upper electrode) suspended above a lower electrode and interposing dielectric layer. An air gap of several microns typically separates the upper membrane from the dielectric layer. The lower electrode comprises a RF signal path, while the upper electrode (whether separate or being the membrane) comprises a RF and DC ground. In the switch “off state”, the air gap between the membrane and lower electrode is sufficient that the upper membrane has an insignificant parasitic capacitance relative to the operating frequency of the switch. When a voltage is applied across the upper and lower electrodes, the electrostatic force pulls the membrane down into contact with the dielectric layer (“on state”). Without a significant air gap, the upper metal membrane, insulator layer, and lower metal electrode form an MIM (metal-insulator-metal) capacitor. This capacitor is designed to achieve sufficient capacitive conductance such that it can capacitively couple, or even short, the RF signal path of the lower electrode to the grounded upper metal membrane. When the applied voltage is released, the restoring force of the membrane metal spring is sufficient to return the membrane to its “off state.” This assumes no secondary effects impede that action, such as charging of the dielectric layer and/or the force of adhesion between the membrane and the dielectric layer.
Electronic switching devices such as RF-microelectromechanical (RF MEMS) switches provide many potential benefits over conventional semiconductor-based switches for controlling and routing microwave and millimeter-wave signals. RF MEMS switches possess very low insertion loss, miniscule power consumption, and ultrahigh linearity. These characteristics make RF MEMS switches ideal candidates for incorporation into passive circuits, such as phase shifters or tunable filters, for implementation in communications and radar systems at RF, microwave, and millimeter-wave frequencies (10 MHz-100 GHz and up).
The insertion into military and/or commercial high frequency systems has been limited by a lack of reliability. In a well-engineered RF MEMS switch, dielectric charging is the main limitation to lifetime, as opposed to mechanical effects. When the switch actuates, a relatively high voltage (30-50 volts) is applied across the relatively thin switch insulator. The resulting electric field induces charge tunneling into the insulator, where they are trapped. As these charges build up, they shift the pull-in and release voltages of the switch. If enough charges become trapped, the operating voltages will shift sufficiently such that the switch will either remain stuck down, or not actuate when desired. In either case, the switch fails to operate properly.
The primary failure mode of conventional prior art RF MEMS capacitive switches is accumulation of charges within the insulator layer made of silicon oxide or silicon nitride materials of the switch, in which charges tunnel into and become trapped within the dielectric. The conventional prior art RF MEMS capacitive switch only recovers from this failure after a sufficiently long period of time (hours to days) during which the trapped charges can diffuse or migrate back to the metal electrodes.
It is, therefore, desirable to provide an improved reliable RF MEMS capacitive switch, or any electrostatically operated MEMS device involving dielectric layers, which overcomes most, if not all of the disadvantages described above.